The field of the invention is variable speed drives for a.c. motors, and particularly, to alternating current sources which provide polyphase, variable frequency, and variable amplitude currents to motor stator windings.
Prior alternating current sources employ solid state switches to produce pulse waveforms that approximate sinusoidal waveforms suitable for application to the motor's stator windings. These are generally divided into two classes: those which employ current source inverters; and those which employ voltage source inverters. A current source inverter receives a controlled d.c. "link" current and switches it between the motor stator windings in such a manner as to approximate the application of polyphase sinusoidal currents of the proper frequency and amplitude. Such an inverter is disclosed, for example, in U.S. Pat. No. 4,400,655. A voltage source inverter on the other hand, receives a d.c. voltage and chops it into a series of voltage pulses which are applied to the motor stator windings. The widths of the pulses are modulated such that the resulting currents produced in the motor stator windings have a sinusoidal shape of the desired frequency and amplitude. Such an inverter is shown, for example, in U.S. Pat. No. 4,469,997.
Both the current source and the voltage source inverters have their respective advantages and disadvantages which are well-known in the art. As a result, both technologies are employed in commercially available motor drive products, with the choice being determined primarily by performance and price consideration.
The present invention solves a problem which is inherent in prior voltage source inverters that employ current regulators. It has long been recognized that at high motor speeds such voltage source inverters do not accurately control sinusoidal motor currents. This has been attributed by some to a loss in gain in voltage source inverters at high speeds, while others relate the problem to the back e.m.f. of the motor. Numerous schemes for solving this problem have been proposed, some of which involve compensation circuitry that is either very complex or that requires detailed knowledge about the particular motor being driven.
The inability of such voltage souce inverters to accurately produce current waveforms of commanded amplitude and phase is particularly troublesome when such inverters are used in a motor drive that relies on controlling the magnetic flux vector. Such vector control strategies require that the amplitude and phase of the sinusoidal current waveform applied to each stator winding be precisely controlled at all times. Only then will the total magnetic flux have the proper magnitude and direction to produce the desired motor torque and speed.